[docs] Add contributing guide

docs/contributing.md

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+# Chatwoot Contributing Guide
+
+Thanks for taking interest in contributing to Chatwoot source code. Before submitting your contribution, please make sure to take a moment and read through the following guidelines:
+
+- [Code of Conduct](https://github.com/chatwoot/chatwoot/blob/master/CODE_OF_CONDUCT.md)
+- [Development Setup](https://docs.chatwoot.com/#/development/environment-setup/ubuntu)
+- [Project Dependencies](https://docs.chatwoot.com/#/development/project-setup/dependencies)
+- [Environment Setup](https://docs.chatwoot.com/#/development/project-setup/environment-variables)
+
+## Pull Request Guidelines
+
+- Assign the corresponding issue to you whenever you start working on it. This helps in resolving collision (mutliple people working on the same item)
+
+- Please raise the PR against master branch
+
+- It's OK to have multiple small commits as you work on the PR - We will squash the commits before merging.
+
+- If adding a new feature:
+  - Add accompanying test case.
+  - Provide a convincing reason to add this feature. Ideally, you should open a suggestion issue first and have it approved before working on it.
+
+- If fixing bug:
+  - If you are resolving a special issue, add `(fix #xxxx[,#xxxx])` (#xxxx is the issue id) in your PR title.
+  - Provide a detailed description of the bug in the PR.
+  - Add appropriate test coverage if applicable.